Barium magnesium aluminate (BAM; [(Ba,Eu2+)MgAl10O17]) has been widely used as a blue-emitting phosphor in Plasma Display Panels (PDPs), three wavelengths fluorescent lamps, or Cold Cathode Fluorescent Lamps (CCFLs) or External Electrode Fluorescent Lamps (EEFLs) for Back Light Units (BLUs) of Liquid Crystal Displays (LCDs).
Unlike PDPs achieving white chromaticity coordinates by separately coating red, green and blue phosphors, CCFLs or EEFLs achieve white chromaticity coordinates by coating an inner part of a glass tube with a slurry obtained by mixing red, green, and blue phosphors in an appropriate ratio, followed by drying and sintering.
In such a phosphor coating for CCFLs, color variation at ends of a CCFL caused by mixing red, green, and blue phosphors with different specific gravities, sizes, shapes, and flowabilities makes the use of the CCFL as a high-quality optical source for displays difficult.
Furthermore, in the above-mentioned slurry coating for CCFLs, the ends and middle portion of the inner wall of a CCFL have a differential thickness of a phosphor layer, thereby leading to brightness variation.
Generally, color and brightness variations at ends of a lamp increase as the length of the lamp increases. Currently, the sizes of display products are increasing, and the lengths of related CCFLs and EEFLs are increasing accordingly. Thus, there is an increasing need to solve the color and brightness variation in CCFLs and EEFLs.
In view of these problems, many studies have been done. However, most studies are related to a slurry coating process achieving brightness uniformity by uniformly forming a phosphor layer on the inner wall of a lamp.
Japanese Patent Laid-Open Publication No. 2001-110309 discloses an aqueous slurry coating method. According to the aqueous slurry coating method, however, considering the fact that currently available CCFLs or EEFLs have mainly a lamp diameter of 5 mm or less, moisture removal after the coating requires a longer time, and twice-coating is required to reduce a thickness difference at ends of a lamp.
Japanese Patent Laid-Open Publication No. Hei. 4-280031 discloses a process for coating a lamp with a slurry containing an organic solvent in such a way that the slurry flows along the axial direction of the lamp from the upper end of the lamp. This process requires primary coating, drying, and sintering; turning upside down of the lamp; and secondary coating, drying, and sintering, to obtain uniform coating at ends of the lamp. According to this process, however, there is formed a crown-type phosphor layer in which the thickness of ends of the lamp is thicker than that of the center portion of the lamp. As a lamp length increases, this phenomenon leads to larger color and brightness variations at ends of a lamp.
Recently published Japanese Patent Laid-Open Publication No. 2004-186090 discloses a method of forming a phosphor layer, which includes preparing a coating solution of a phosphor-binder slurry with a viscosity of 30 cP or more using an organic solvent; applying the coating solution onto an inner wall of a lamp using a suction pipe by once coating; drying the coating with a flowing gas; and sintering the dried coating. This method is the most widely used method for manufacturing CCFLs but is insufficient in achieving less color variation and better brightness uniformity along the lengthwise direction of a glass tube.
The above-described prior arts for enhancing brightness uniformity at ends of lamps have been focused on development of better slurry preparation or coating methods rather than on development of better phosphors.
In this regard, the present invention provides a BLU lamp for LCD which offers less color variation at ends of the BLU lamp by improving the characteristics of phosphors. For this, the present invention provides a novel BAM phosphor in which phosphor particles are surface-modified with metal oxide nanoparticles, and a method for preparing the same.
The surface-treatment of phosphors with metal oxide was conducted for various different types of purposes. Japanese Patent Laid-Open Publication Nos. Hei. 11-172244, Hei. 9-231944, 2002-348570, 2003-147350, 2003-226872, 2004-244604, etc. reported that brightness degradation by vacuum ultraviolet radiation was reduced by forming a rare earth oxide film to a thickness of 5 to 100 nm by surface coating of phosphor particles with nitric acid and metal oxide such as La2O3, Y2O3, SiO2, and Gd2O3 (Japanese Patent Laid-Open Publication No. Hei. 11-1722440), or by forming a rare earth carbonate film by surface coating of phosphor particles with rare earth metal carbonate (Japanese Patent Laid-Open Publication Nos. 2003-147350, 2003-226872, and 2004-244604). These patent documents mentions color or brightness variation at ends of a lamp but is silent about initial brightness degradation of a phosphor due to coating. The formation of a protection film on a surface of a phosphor induces a change in emission efficiency according to a coating amount. As the coating amount increases, reduction in emission efficiency increases but brightness maintenance rate increases. Furthermore, a coating material serves as a protection film but may serve as a binder, thereby causing the agglomeration of phosphor particles. The agglomerated phosphor particles may not form a uniform coating film in actual use due to poor dispersion property, thereby leading to unevenness in chromaticity coordinates and brightness.